Stabilizing system for cathode ray tube

ABSTRACT

The invention pertains to a video display apparatus which includes a cathode ray tube having at least one electron gun and means for deriving a source of excitatory voltage signal representative of picture information. In accordance with an embodiment of the invention, there is provided a system for stabilizing the display intensity of the electron gun comprising means for amplifying the voltage signal and a constant current means including resistive means coupling the amplified voltage signal to the cathode of said electron gun, the resistive means having an impedance which is substantially greater than the input impedance of the cathode. In another embodiment of the invention, amplifier means are provided for applying the excitatory voltage signal to a control grid of the electron gun and a feedback signal generating means is provided for sensing the beam current of the electron gun and generating a feedback signal as a function of the sensed current. The amplifier means is modulated in accordance with the generated feedback signal.

BACKGROUND OF THE INVENTION

This invention relates to improvements in video display apparatus and,more particularly, to a system for stabilizing the display intensity or"color temperature" of a cathode ray tube. The subject matter of thisinvention is related to subject matter disclosed in copending U.S.application Ser. No. 572,169 and now U.S. Pat. No. 4,012,775 of C. W.Smith, filed of even date herewith and assigned to the same assignee.

Conventional television display systems employing kinescope cathode raytubes are subject to performance degradation resulting frominstabilities in the operating characteristics of the kinescope or thecircuits which drive or bias the kinescope. Prior techniques have beendeveloped which serve to stabilize the signals driving a kinescope. Forexample, the drive voltages applied to the cathodes of a color kinescopecan be stabilized using a feedback scheme; e.g., circuitry whichperiodically senses the drive voltage at input "black" and "white"levels of operation and corrects for deviations from standard referencevoltages by gain adjustment. DC voltages applied to the kinescope canalso be stabilized by using precise voltage regulation techniques.

There remains, however, the recognized problem of kinescope electron gundrift which manifests itself as a drift in screen color temperature in athree gun color kinescope. As the electron guns age, their generatedbeam current per unit of applied voltage (which can be considered atransconductance function) varies, the variations being generallynon-uniform in the three different guns. This is a cause of noticeableand undesirable drifts in the display screen color.

The major sources of drift are: aging or long term variations caused bya gradual decrease in cathode activity, not necessarily constant oruniform for each cathode; and cathode operating temperature. Therelatively long term variations in emission are caused by filamentvoltage changes and heat build-up in the gun area, generally a functionof how many hours a display tube has been operating. Dynamic heating ofeach gun depends on the ratio of gun currents drawn to provide thecolored picture being instantaneously presented. For example, a longpersisting mostly red field causes red gun current almost exclusively,thereby causing an unbalanced heating of the red cathode, which changesits emission characteristics to a different degree than the othercathodes, this change remaining until relative cooling occurs.

Cathode thermal current, I.sub. th, is represented by the Dushmanequation:

    I.sub.th =  SA.sub.0 T.sup.2 e.sup.-b.sbsp.0.sup./T amperes

where S and A₀ are constants and b₀ = Dushman constant ≈ 11,600° for anoxide coated cathode

The derivative of the natural logarithm of this equation gives thechange in emission with respect to temperature change: ##EQU1## Thetemperature of the CRT cathode is approximately 1,160° K, which yields

    dI.sub.th/ I.sub.th = 12dT/T

typical ambient temperature variations, such as in a display monitor,are about 40° C, so that the net change of gun current is of the orderof

    12 · 40/1,160 ≈ 40%

Therefore, a 1° C change in cathode temperature yields about a 1% changein gun current, if the gun is fixed bias and not near cut-off.

It is an object of the present invention to provide a stabilizing systemwhich overcomes the problems set forth.

SUMMARY OF THE INVENTION

The invention pertains to a video display apparatus which includes acathode ray tube having at least one electron gun and means for derivinga source of excitatory voltage signal representative of pictureinformation. In accordance with an embodiment of the invention, there isprovided a system for stabilizing the display intensity of the electrongun comprising means for amplifying the voltage signal and a constantcurrent means including resistive means coupling the amplified voltagesignal to the cathode of the electron gun, the resistive means having animpedance which is substantially greater than the input impedance ofsaid cathode.

In another embodiment of the invention, amplifier means are provided forapplying the excitatory voltage signal to a control grid of the electrongun, and a feedback signal generating means is provided for sensing thecathode current of the electron gun and generating a feedback signal asa function of the sensed current. The amplifier means is modulated inaccordance with the generated feedback signal.

Further features and advantages of the invention will become morereadily apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a color television display kinescope;

FIGS. 2A, 2B and 2C are block diagrams of embodiments of the inventionwhich utilize periodically applied test signals;

FIG. 3 is a schematic representation of an embodiment of the inventionwhich employs a "constant current" technique;

FIG. 4 is a schematic representative of another embodiment of theinvention employing a differential amplifier; and

FIG. 5 is a schematic representation of another embodiment of theinvention employing direct cathode temperature sensing and heatercontrol.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a simplified diagram of a colortelevision display cathode ray tube or kinescope 10 as driven byexcitatory video voltage signals designated R, G and B, these signalshaving typical ranges of about 100 volts peak-to-peak. The kinescope 10has three electron guns, each including a cathode and associated grids.For clarity of illustration, only one of the three guns, designated byreference numeral 11, is represented in some detail, but it will beappreciated that two other complete electron guns (indicated in theFIGURE by only the two dashed cathodes coupled to the G and B inputs)are normally provided and are substantially identical to the gun 11.Hereinafter, and in the description of the embodiments of the invention,the circuitry associated with only one electron gun in a given kinescopewill be described for illustrative clarity, but it will be understoodthat if the kinescope has two or more guns, similar circuitry can beemployed in conjunction with the remaining guns.

The electron gun 11 comprises a cathode 21 and first, second and thirdgrids, 22, 23, and 24, which are sometimes designated as "grid 1," "grid2," and "grid 3," or as the "control electrode," the "acceleratingelectrode" and the "focusing electrode," respectively. Generatedelectrons impinge on an anode 25 near display screen 26 which is coatedwith an electron-sensitive phosphor as is conventional in the art.Typical voltages applied to the cathode, grid 1, grid 2, grid 3 and theanode are about 235 volts, 150 volts, 700 volts, 5000 volts and 25,000volts, respectively. In alternate modes of operation, the excitatoryvoltage input signal may be applied to a control grid with the remaininggrid and cathode voltages being set at appropriate values.

Referring to FIG. 2A, there is shown an embodiment of the inventionwhich comprises a system for stabilizing the display intensity or "colortemperature" of the electron guns in a kinescope 30. An excitatoryvoltage signal at input terminal 31, which may be the R, B or G signalin a color system or the luminance signal in a black and white system,is coupled through an adder 41 and DC restorer circuit 42 to grid 1 ofthe kinescope 30. Cathode 43 is coupled through a resistor R₁ to groundreference. A bias voltage is applied to grid 2 via a voltage amplifier44 which receives a signal on a line 58A which determines the level ofthe bias voltage applied to grid 2. Suitable focus and anode voltagesare applied to grid 3 and the anode from sources not shown.

The vertical and horizontal synchronizing signals of the compositetelevision signal, available in the television receiver, are applied toa line counter 51 which is adapted to count horizontal scanlines of thetelevision field and to be reset to zero at the end of each televisionfield. The counter generates a first output on a line 51A during thescanlines 15-17 of each television field and a signal on line 51B duringlines 18-20 of each television field, all of the lines 15-20 occuringduring the vertical blanking period. The signal on line 51A enables agate 55 and also enables a sample-and-hold circuit 56. The signal online 51B enables a gate 57 and a sample-and-hold circuit 58. The gates55 and 57 respectively receive voltages at reference "black level" and"white level." The outputs of gates 55 and 57 are coupled over lines 55Aand 57A, respectively, to inputs of the adder 41.

Operation of the system of FIG. 2A is as follows: During lines 15-17 ofthe vertical blanking interval the gate 55 is enabled so that blacklevel voltage is coupled through adder 41 and circuit 42 to grid 1. Withthis voltage applied to grid 1 the cathode current should ideally have acertain nominal value that does not vary with the tube life or cathodetemperature but, as indicated above in the Background, this is notgenerally the case in actual practice. The actual cathode current issampled across resistor R₁, and a voltage representative of this currentis coupled to the sample-and-hold circuit 56 which is enabled to samplethe voltage across resistor R₁ during the lines 15-17. The circuit 56holds the sampled voltage through the subsequent video field and couplesthe held voltage to circuit 42 via line 56A, this voltage serving toadjust the DC reference level of the output of circuit 42. In thismanner, the voltage on line 56A controls the bias level at grid 1 so asto correct for any variations in the cathode current at nominal blacklevel. Thus, for example, if at some point in operation the cathodecurrent for a "black level" input voltage is lower than its nominalvalue, the voltage drop across sampling resistor R₁ will also be low.This will decrease the output of sample-and-hold circuit 56 fed tocircuit 42 which, in turn, will cause the bias level at grid 1 todecrease (typically, to a less negative value with respect to thecathode). A lesser negative bias level on the control grid 1 will, inturn, cause a proportionate increase in the electron current flowingfrom cathode 43; the desired result.

Similarly, during lines 18-20 of the vertical blanking period "whitelevel" voltage is applied via adder 41 and amplifier 42 to grid 1, andduring this time the cathode current is sampled by circuit 58 which isenabled to sample by the signal on line 51B. During the remainder of thetelevision field, the bias voltage applied to grid 2, via voltageamplifier 44, is a function of the voltage which has been sampled bycircuit 58. For example, in an instance where the cathode current sensedat a "white level" voltage input is lower than the nominal value, theresultant low voltage sampled by circuit 58 will cause the grid 2accelerating voltage to decrease. This causes the sampled voltage atblack level to appear too negative (when next sampled during thesucceeding vertical blanking interval) which, in turn, results in adecrease in grid bias by the black level circuit causing the desiredincrease in beam current over prior conditions, as previously described.

The embodiment of FIG. 2B is similar to that of FIG. 2A except that theoutput of sample-and-hold circuit 58 (which is a measure of the sampledwhite level current) is coupled to an analog multiplier circuit 59,which is in series with DC restorer circuit 52. In this embodiment,corrections resulting from both the white level and black levelmeasurements are achieved via grid 1, with operation otherwise beingsubstantially as described above.

In the embodiment of FIG. 2C the electron gun is driven by applicationof the video signal to the cathode 43 via a complementaryemitter-follower 120 which comprises NPN transistor 121 and PNPtransistor 122. (The system to the left of blocks 42 and 56 is the sameas in FIG. 2B). The transistor emitters are coupled to the cathode 43 ofkinescope 30 and the transistor bases receive the video signal from DCrestorer circuit 42. The collector of transistor 121 is coupled to asuitable bias voltage, e.g., 150 volts, and the collector of transistor122 is coupled to ground reference potential through sampling resistorR₁.

In operation, during the lines 15-20 the test signals are applied viacircuit 42 and cathode 43 is driven while the cathode current is sampledby resistor R₁, a typical value for which is 1K ohm. Transistor 122 is"on" during the white level test signal (output of circuit 42 about 25volts) and the black level test signal (output of circut 42 about 125volts), and the gun current-representative voltages sampled acrossresistor R₁ are coupled to the appropriate sample-and-hold circuits aspreviously described. During the active portion of the television fieldthe analog multiplier 59 and DC restorer circuit 42 apply appropriatecorrections, with transistor 122 normally "on." During rapiddarker-to-lighter transitions of the video signal the transistor 121turns momentarily "on" and the tube capacitance and stray capacitance(collectively represented by C in the FIGURE) can be thought of ascharging. Diode D₁ prevents inordinate voltage drops across R₁ duringthe active picture area when R₁ is not used for sampling.

In the embodiment of FIG. 3 the video voltage signal at terminal 61 iscoupled to cathode 71 of a kinescope 75 by the parallel combination ofcapacitor 62 and amplifier 63 in a series with resistor R₂. Amplifier 63comprises transistors 64 and 65 and has a voltage gain of about 5 and anoutput capability of about 500 volts. The resistor R₂ is selected to besubstantially greater than the input impedance of the cathode 71 andpreferably has a resistance at least five times higher than the cathoderesistance. Since the effective cathode resistance is the inverse of thegun transconductance (about 8.6 micromhos), a suitable value for R₂ isof the order of 600K ohms. Accordingly, the amplifier 63 in conjunctionwith resistor R₂ operates as a so-called "constant current" source,which effectively transforms the voltage signal at terminal 61 to acurrent source input to the cathode 71, this current source input beingrelatively insensitive to variations in the kinescope characteristics.Since normal wiring capacitance and electron gun interelectrodecapacitance render high frequency response impractical in a highimpedance amplifier drive, the higher frequency portions of the videosignal are shunted across the amplifier by capacitor 62 which may have atypical value of about 0.05 microfarads. The higher frequency signalsarrive at substantially the same relative level as the low frequencies,thereby preserving their relationship. This is because the lowerfrequency signals are amplified by a factor of 5 and then undergo aone-fifth loss by virtue of the voltage divider action of resistor R₂and the cathode impedance.

FIG. 4 shows a further embodiment wherein the video voltage signal at aninput terminal 81 is applied to one input of a differential amplifiercomprising transistors 82, 83 and 84. The output stage 84 drives thegrid 1 electrode of kinescope 90 through series peaking inductor L₁ andshunt peaking inductor L₂. The cathode 91 of kinescope 90 is coupled toground reference potential through resistor R₃ which is used tocontinuously monitor the cathode current, the line 89 coupling a voltagerepresentative of the cathode current to the other input of thedifferential amplifier; viz, the base of transistor 83.

In operation, the voltage developed across resistor R₃ is proportionalto the cathode current. This voltage, for a stable transconductance,should be in a stable relationship with respect to input voltage atterminal 81, and R₃ is selected empirically at a value, typically about2K ohms, which generates a sample voltage nominally equal to the inputvoltage at terminal 81. When a deviation exists between the inputs totransistors 82 and 83, the output of the differential amplifier adjustsup or down to correct for the difference, thereby adjusting control ofthe drive to grid 1 and correcting for drifts in the kinescopetransconductance.

A characteristic of the circuit of FIG. 4 is that it linearizes theelectron gun transfer function which normally is non-linear, thenon-linear function conventionally being known as the "gamma" of thekinescope. Television video signals are conventionally precorrected forthe gamma of the kinescope. In a color kinescope the gamma may bedifferent for each gun, making it difficult to match the effective lightoutput attributable to each gun over the grey scale; a problem known as"tracking" in the prior art. The present invention allows use of aninverse gamma circuit (which eliminates the precorrection in theconventional television signal) and the linearized gun transferfunctions reduce tracking problems.

The invention has been described with referenec to particularembodiments, but it will be understood that variations within the spiritand scope of the invention will occur to those skilled in the art. Forexample, the circuits of the "constant current" generator FIG. 3 or thedifferential amplifier of FIG. 4 may be of other suitable forms. Also,in the embodiment of FIG. 2, sampling could be achieved during anysuitable blanking or active period. The beam could be deflected off thetube face during sampling time to avoid displaying the trace during thistime. Finally, stabilization of cathode temperatures could be achieveddirectly, such as by providing heater/thermistor stabilization circuitsfor each cathode. A suitable circuit is shown in FIG. 5 wherein anegative temperature coefficient thermistor 101 is attached to thecathode metal. V₀ is a precision voltage source providing a voltagetypically in the range 5-12 volts and R₀ is selected as beingsubstantially equal to the resistance of the thermistor at nominalcathode temperature. If the cathode becomes unduly hot, the resistanceof thermistor 101 will decrease which, in turn, causes the voltage atterminal 103 to decrease. This results in a decreased output ofoperational amplifier 102, so that the cathode heater drive is reduced,as desired. Insufficient cathode temperature can be seen to cause theopposite effect.

We claim:
 1. In a video display apparatus which includes a cathode raytube having at least one electron gun and means for deriving a source ofexcitatory voltage signal representative of picture information; asystem for stabilizing the display intensity of said cathode ray tubecomprising: means for amplifying said voltage signal; constant currentmeans including resistive means coupling said amplified voltage signalto the cathode of said electron gun, said resistive means having animpedance which is substantially greater than the input impedance ofsaid cathode; and capacitive means in parallel with said constantcurrent means for shunting high frequency voltage signals across saidconstant current means; whereby the current applied to said cathode issubstantially dependent only upon said voltage signal and is relativelyinsensitive to variations in the characteristics of said cathode.
 2. Thesystem as defined by claim 1 wherein said resistive means has animpedance which is more than twice as great as the input impedance ofsaid cathode.
 3. In a video display apparatus which includes a cathoderay tube having a plurality of electron guns and means for deriving arespective plurality of excitatory voltage signals representative ofcolor picture information; a system for balancing the screen colortemperature of said cathode ray tube, comprising: a plurality of meansfor respectively amplifying said plurality of voltage signals; aplurality of constant current means, each including resistive meanscoupling an amplified voltage signal to the cathode of its respectiveelectron gun, each of said resistive means having an impedance which issubstantially greater than the input impedance of the cathode of itsrespective electron gun; and a plurality of capacitive means in parallelwith said constant current means for shunting high frequency voltagesignals across said constant current whereby the current applied to eachcathode is substantially dependent only upon its respective voltagesignal and is relatively insensitive to variations in thecharacteristics of the cathode.
 4. The system as defined by claim 3wherein each of said resistive means has an impedance which is more thantwice as great as the input impedance of the cathode of its respectiveelectron gun.